The present invention relates to heat exchange systems employing an aqueous coolant. More specifically, the present invention relates to a system and method for reducing the amount of bleed-off required in such heat exchange systems.
In many heat exchange systems and processes, evaporation of aqueous coolant can cause concentration of dissolved salts, precipitation of salts, and concentration of suspended solids. This material concentration in the aqueous, coolant can cause various problems as further discussed below.
Cooling towers are frequently used as cooling devices in heat exchange systems. In such systems the heat transferred to the aqueous coolant causes the water to vaporize and thus increases the concentration of the ions and solids in the aqueous coolant. Also, there is loss of water and dissolved and suspended solids due to windage.
The concentration of dissolved ions, if left uncontrolled, quickly exceeds the solubility of the certain ions and therefore salts precipitate, e.g., CaCO.sub.3. Such salt deposition causes various problems in boiler and cooling tower systems, i.e., it increases corrosion and scale buildup. The salt deposition also increases maintenance costs, reduces equipment life and substantially increases energy costs due to the insulating properties of the deposits which impede efficient heat transfer.
Even with dissolved ions of high solubility, such as sodium or potassium, corrosion of metals in the boiler or circulating cooling systems is increased because of the increased corrosivity due to increased electrolyte causing increased electron flow, thus accelerating the electrochemical action of metals and corrosive agents in the water. Thus, heat exchangers, boilers, distribution pipes, and pumps are commonly corroded by the concentration of such metallic elements in the system. Corrosion of these components and subsequent deposition of the corrosion by-products also leads to unscheduled down time, increases maintenance costs, reduces equipment life and substantially increases energy costs.
Furthermore, in addition to deposit formation and corrosion, high concentrations of dissolved ions reduce the surface tension of water, causing foam within the system. Such foam formation interfers with the heat transfer process.
Suspended solids can directly deposit onto the heat transfer surfaces and thus cause the same problems as with salt deposits. Suspended solids also increase corrosion by promoting concentration of corrosive ions beneath the deposits of such suspended solids.
Various techniques have been employed to control suspended solids and dissolved ions in heat transfer systems. Traditionally, such control has been achieved by removing some of the concentrated aqueous coolant and diluting it with make-up water of a lower dissolved ion and suspended solids concentration. This process is called bleed-off. Since the aqueous coolant normally contains various inhibitors for scale buildup, corrosion, and microbiological growth, these materials are removed during bleed-off. Therefore, new inhibitors must be added in the make-up water to insure that they are maintained at proper levels and therefore this bleed-off method substantially increases chemical usage. In addition, the bleed-off process results in substantial water usage. Furthermore, because the bleed-off water contains not only the inhibitors, but also the highly concentrated ions and suspended solids, the bleed-off water poses potential problems in terms of both water pollution and solid waste control.
Other techniques more complex and more costly than simple bleed-off have also been tried. For example, softening of the aqueous coolant by employing a sidestream aqueous water softener has been employed in an attempt to reduce the amount of bleed-off necessary in a circulating heat exchange system. For example, one such technique employs a sidestream softener and filter in order to remove suspended solids and certain ions which form solids from the water coolant circulating in the system. This system employs a by-pass lime softening step to accomplish such objective. Thus, in the process, only the recirculating cooling water is treated, and some ions are removed by precipitation while other ions are replaced by different ions (for example, calcium is replaced by sodium). This system removes chemicals from the system including some inhibitors, which may be included in the circulating water. Moreover, the system generates relatively large amounts of solid waste which must be disposed. Furthermore, the lime softening increases the alkalinity of the circulating cooling water, requiring the addition of relatively large amounts of acid, such as sulfuric acid, to the circulating cooling water to maintain the proper pH of the coolant. This acid addition, in itself, results in additional ions in the system along with the problems associated therewith. All of the above contribute to the capital and operational costs of such a system.
A similar technique is described in Lawlar U.S. Pat. No. 3,805,880. However, the Lawlar system includes a water softening treatment of the make-up water, employing, for example, a sodium zeolite water softener. Thus, the Lawlar system does not reduce the total number of ions in the circulating aqueous coolant system; but rather, it replaces certain low solubility cations such as calcium with higher solubility cations such as sodium. The Lawlar system thus will not avoid the problems resulting from high sodium and potassium contents, i.e., high alkalinity, the associated corrosive character thereof, the need to add acid to reduce such alkalinity and the associated ion build-up caused by acid addition. Moreover, while the Lawlar system does reduce bleed-off, it gives no regard to reducing or treating silica, which by itself will limit the cycles of concentration with the Lawlar system even with make-up water with moderate silica levels. Furthermore, in the only example in the Lawlar patent, only about 10 cycles of concentration were obtained.
Applebaum in U.S. Pat. No. 2,807,582 discloses a system for treating boiler feed water for high pressure boilers in which the feed water is first passed through a cold split stream hydrogen-sodium zeolite plant which reduces hardness to practically zero and reduces alkalinity to about 15 ppm, converting the alkalinity to free CO.sub.2. The water is then degassified and pumped through anion exchange units, which are of the strongly basic type, thus greatly increasing the alkalinity of the cooling water. The patent even notes that the anion exchange effluent is too high in sodium hydroxide content to be used for boiler feed purposes. Therefore, sulfuric acid is added to neutralize the excess base. Thus, the Applebaum system requires a high capital cost for the various ion exchange treatment steps and is relatively complex in nature. Moreover, although the Applebaum system does remove some ions from the system by the split stream hydrogen zeolite portion of the first step, the Applebaum system still substitutes some Na.sup.+ for hardness values in the water. Furthermore, the strong zeolite materials employed by Applebaum require about 6 times the amount of regenerant (e.g., acid for the acid zeolite) to regenerate the active forms.